1. Field of the Invention
The invention relates to a display tube comprising an evacuated envelope consisting of a display window, a cone, and a neck, and an electron gun for generating an electron beam which is focused to form a spot on a display screen provided on the inside of the display window, said spot being deflected over said display screen in two substantially mutually perpendicular directions by deflection fields of a system of deflection coils. It particularly relates to means for shaping such deflection fields so as to minimize defocusing of the electron beam over the display screen.
2. Description of the Related Art
Such a display is used, for example, in a device for displaying symbols and/or figures generated, for example, by a computer. Such a display tube is also termed a D.G.D.-tube (D.G.D.=Data Graphic Display).
Such a display tube is known, for example, from "Philips Data Handbook", Electron Tubes, part 8, July, 1983, Monitor Tubes.
In such tubes deflection coils are used in which the generated line deflection field, but usually also the field deflection field, is made pincushion-shaped to minimize the frame distortion. As is known, however, such pincushion-shaped deflection fields also influence the shape of the electron beam and hence the shape of the spot on the display screen which is the result of said beam. In D.G.D.-tubes it is highly desirable for the spot dimensions not to vary too much over the whole display screen, so that symbols and figures are displayed substantially equally sharply in the center, in the corners and at the edge of the display screen. In order to reduce deflection defocusing it is known to use field shapers in colour display tubes (see, for example, U.S. Pat. No. 4,346,327, PHN 9052).